Estrogen receptor-positive (ER+) luminal breast cancers, which account for 75% of all breast cancer cases, are the subtype of breast cancer most likely to metastasize to bone, forming incurable lytic lesions that are, in a majority of case, also ER+. In contradistinction, prevailing pre-clinical models investigating the pathogenesis and treatment of breast cancer bone metastases (B-MET) employ ER- basal subtypes of human breast cancer cells. In the absence of standard models of ER+ luminal B-MET, basic questions regarding the biology of these tumor cells within the bone microenvironment cannot be queried, including a possible role for tumor cell estrogen signaling in promoting metastasis progression within the bone microenvironment. The objective of this proposal is to utilize in vivo selection and commonly studied ER+ luminal human breast cancer cell lines to establish robust pre-clinical models of ER+ lytic B-MET which will then be queried, using functional genomics, to examine the pathogenesis of bone metastasis in this setting, comparing and contrasting these findings with those previously obtained in ER- basal cell models. In Specific Aim 1, distinct subpopulations of ER+ cells with a high metastatic potential for formation of lytic B-MET will be isolated by in vivo selection following intracardiac (IC) inoculation of heterogeneous populations of low-metastatic human ER+ luminal breast cancer cells (MCF-7, ZR-75-1, or T47D) into nude mice. These studies will take advantage of, and build upon, unique osteotropic ER+ MCF-7-derived cell lines already isolated by our laboratory using this approach. In Specific Aim 2, functional genomic studies, comparing gene expression in cells specifically selected for their bone tropism during the final stage of metastatic progression (osteotropic cells, as described in aim 1) vs. low- metastatic cells from which they are derived, will be undertaken to identify genes whose constitutive or estrogen-stimulated expression is associated with tumor progression and osteolysis in the bone metastatic niche. In vivo testing to determine estrogen-dependency and site-specificity of enhanced tumor progression for these osteotropic cells will further aid in interpreting the functional significance of genes associated with the osteolytic ER+ luminal B-MET phenotype. Success in achieving these aims is likely given that: 1) we are recapitulating the experimental approach used to determine the pathogenesis of ER- basal cell breast cancer B-MET, and 2 ) we have already isolated osteotropic ER+ MCF-7 cells and identified osteolytic B-MET in nude mice inoculated IC with ER+ T47D cells in preliminary experiments. The significance of these studies lies in their novelty, as ER+ luminal models of lytic B-MET have not been developed for interrogation in functional genomic studies, and their clinical relevance to breast cancer, as these studies address a common clinical situation for which there is no cure. Our ultimate goal is to increase survivorship in women diagnosed with breast cancer by improving strategies to prevent and treat ER+ luminal breast cancer B-MET through the current and future use of the novel tools and information acquired in the R03 studies proposed here.